DALLAS (AP) — Southwest Airlines Co. said Tuesday it finished inspecting 79 older planes and found that five of them have cracks in the aluminum skin.

That's two more than the airline found by Monday afternoon.

Boeing said Tuesday it is directing Southwest and other airlines that own certain older 737s to conduct electromagnetic inspections of a portion of the roof once the planes have made a certain number of trips.

A Boeing 737-300 sprung a hole in the roof during a Southwest flight on Friday, forcing an emergency landing, In response, Southwest grounded 79 planes of the same model over the weekend.

Boeing project engineer Paul Richter said Southwest will have to replace an 18-inch section of overlapping aluminum panel called a lap joint on the planes with cracks in the skin. Southwest said the five planes with "minor subsurface cracking" will remain grounded until they can be repaired.

The plane that ripped open 34,000 feet above Arizona will face "obviously a much larger repair," Richter said.

Southwest has built its operation around the 737, which it uses for frequent takeoffs and landings each day.

Richter said there was nothing in the way Southwest operates that contributed to the lap joint tearing open on Friday's flight in Arizona.

Southwest said operations were returning to normal Tuesday after nearly 700 flights were canceled Saturday through Monday.

Federal officials say they will order emergency inspections of some older 737s for cracks in the fuselage like the ones on the Southwest jets. The Federal Aviation Administration order was expected on Tuesday.

Southwest said it had already complied with the new order by grounding and inspecting planes over the weekend.

Richter, the Boeing engineer, said his company is directing airlines that own certain older 737s, including Southwest, to conduct electromagnetic inspections of a 50-foot section of each plane's crown or roof after 30,000 takeoffs and landings. Boeing is also recommending inspections every 500 cycles after that, which Richter called a "rare" measure but a necessary one because of the unexpected metal fatigue in the Southwest jet that ripped open.

A cycle is one takeoff and landing. Boeing had expected airlines wouldn't have to examine the lap joint on those planes until they had flown 60,000 cycles.